It's a little-understood (and poorly named) custom function. The D700 has the ability to sync. with flash in three different modes:

-Normal, in which the sync. speed is limited to 1/250th, the camera's highest normal sync. The shutter opens during the peak intensity phase of the flash burst.

-"1/250th FP", in which the camera will sync. normally up to 1/250th, and then switch to FP (rapid burst) to provide a nearly continuous, evenly integrated light for the entire duration of shutter travel, and used above 1/250th but drastically reducing distance range and increasing load on the flash unit.

-"1/320th FP", a little-known choice that most people gloss over, misunderstand, and is poorly named (imho - would have been better to be called 1/320th hedge sync.). At this setting, the timing of the flash burst is altered such that the camera can sync. at up to 1/320th second WITHOUT FP being used. The disadvantage is slight (approx one stop) loss of illumination, and some minor corner/edge darkening of the frame in some cases. It seems to be similar to one of the "hypersync" modes offered by Pocketwizard brand remotes.

Anyone use the 1/320th sec FP setting? Seems like a great way to get a little more shutter speed leeway when performing fill-flash with a telephoto lens and a moving subject that needs to be frozen (assuming the light loss and edge darkening isn't an issue).

Trivia fact for those who are familiar with the F5: it had a similar setting that allowed TTL flash sync. without resorting to FP mode up to 1/300th sec.

An undeniable paradox: To think that there is any such thing as an absolute rule is at worst naïve, and at best, shortsighted. There is no such thing as an always-true, all context- or situation-salient, absolute rule that always holds true…including this one!

I have my D700 set to 1/320th. I did so because, like you, I thought it was a good way to get the most out of the camera. I did not fully appreciate the slight loss of illumination issue.

Anyway, your post made me curious so I decided to do a little adhoc testing. Unfortunately, it is heavily overcast here today and likely to be so for the next few days. With the sky acting like a giant softbox, I was not able to force the camera into high speed synch.

As a fall back, I had a look through some of Joe McNally's books to see what he has to say. Joe seems to have his camera's set to 1/250th and is silent on the use of 1/320th.

Finally, I re-read the manual. As you already know, that was of no help at all.

I would also like to hear what others think about this. Have you considered reposting in the flash forum?

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I have always left my D300 and D700 in 1/320 FP mode whenever I wanted the FP option - and that is almost all the time.

I never understood the reason for 1/250 FP - never understood why I would want to shut out the option of that extra fractional stop. If the camera offered 1/1000 FP that would be better still!

It is not clear to me at the moment what difference, if any, occurs when a 1/320s speed is used when the camera is set to 1/250s FP. Presumably it shoots using much less efficient FP bursts when it doesn't have to do that, and could use the more efficient standard flash sync. I might have tested it years ago and I guess I should refresh my memory .

So I think the question should be "who doesn't use 1/320 FP" because I think a good reason would be hard to come by

I was not aware of (or recall noticing) any edge darkening issues - I'll have to test that some time.

When I am shooting wildlife with a long lens I usually keep it to 1/250s because I don't want to lose that stop, or some fraction of a stop at 1/320s even in 1/320s FP mode. I usually want all the light I can get, if only to minimize the recycle time. Any edge darkening would rarely be an issue with my birding since I am typically doing deep crops anyway.

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Neil, I thought 1/320 fp allowed a shoe mounted flash, the SB-800 or 600 in my case to sync all the way to 1/8000. Is this the case? I like using fill flash outdoors to eliminate shadows.

mVs
D2H, D2x, D300s, D700 & D800

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>I thought 1/320 fp allowed a shoe mounted flash, the
>SB-800 or 600 in my case to sync all the way to 1/8000. Is
>this the case? I like using fill flash outdoors to eliminate
>shadows.


Yeah, but so does the 1/250 FP mode.


They are exactly the same above 1/320th. (And at 1/250th or below.)
It's just within that narrow window between 1/250th and 1/320th that the two modes differ. You get a little more output (and faster recycling) in that range of speeds using the 1/320 option because the flash doesn't have to use multiple bursts (FP). The only reason to use 1/250 would be if you're afraid of the slight darkening that could occur along one side of the frame or need the full output. Note: a flash meter won't provide accurate metering when using the 1/320 option, either. In this case, the shutter speed does affect the flash exposure (and you still lose some output versus what you'd get at 1/250 because the shutter has already started to pass part of the frame by while the flash is still exposing the frame).

But when it comes to handholding a 300mm lens for fill flash, the slightly higher shutter speed can make the difference in a sharp picture.

An undeniable paradox: To think that there is any such thing as an absolute rule is at worst naïve, and at best, shortsighted. There is no such thing as an always-true, all context- or situation-salient, absolute rule that always holds true…including this one!

I believe the 1/320 still uses a single burst and not FP; it just optimizes the sync. timing in such a way as to barely eek out a little more shutter speed. You lose illumination, but not as much as you do in FP mode. If you're set to 1/250 FP, the camera will use FP at 1/320th and you'll lose even more light. That's the difference as I understand it.

To learn more about how it works, I suggest reading up on "hypersync"; I believe it's a similar idea.

Many pros who use studio flash units wouldn't want to exceed the maimum sync. speed, because their studio units sync. at much slower speeds (due to their output versus time curve?). That can be a full stop slower than the normal sync. speed in some cases.

An undeniable paradox: To think that there is any such thing as an absolute rule is at worst naïve, and at best, shortsighted. There is no such thing as an always-true, all context- or situation-salient, absolute rule that always holds true…including this one!

The weather has improved here and I was able to take these test shots. One was without flash for reference and the other two were at 1/250 and 1/320 synch speed. The top photo was shot at 250th and the second at 320th.

Unfortunately, I can see any edge differences between the two shots. The rabbit in the second shot does seem better exposed, but that may be some extraneous factor. Am I missing something?

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Your test shows that for fill flash in which there is a bright background covering that edge part of the frame, there is no real difference in exposure between the two shutter speeds.

I would test it again, but in a situation in which there is low ambient light and the flash illuminated object takes up the entire frame and has a uniform tonality (such as a gray card) if you really want to see the potential edge darkening.

I assume you were at the same aperture and ISO for all frames. These need to be held constant.

An undeniable paradox: To think that there is any such thing as an absolute rule is at worst naïve, and at best, shortsighted. There is no such thing as an always-true, all context- or situation-salient, absolute rule that always holds true…including this one!

I don't have a gray card handy so I can't do the test you propose. I do wonder though whether I would have been able to get the shutter speed high enough to force the camera into auto fp mode?

I normally only use auto fp mode for fill flash in high contrast situations. Therefore I wonder whether edge darkening under different circumstances would be a real issue for me.

Leaving that aside, you original question is still a very good one.

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I saw mention of 1/320th FP in the manual but didn't give it much thought. If the flash sync speed is 1/250th sec, then for anything faster than this all of the sensor is not exposed at the same time. So what possible altering of the (single) flash burst can get around this basic fact?

Barry